The present invention generally relates to a calibration method for semiconductor process equipment and more particularly, relates to an apparatus and a method for detecting the tilt angle of a wafer platform in a semiconductor process machine.
Ion beam implanters are used to implant or xe2x80x9cdopexe2x80x9d silicon wafers with impurities to produce n or p type doped regions on the wafers. The n and p type material regions are utilized in the production of semiconductor integrated circuits. Implanting ions generated from source materials such as antimony, arsenic or phosphorus results in n type material. If p type material is desired, ions generated with source materials such as boron, gallium or indium are typically used.
The ion beam implanter includes an ion source for generating positively charged ions from ionizable source materials. The generated ions are formed into a beam and accelerated along a predetermined beam path to an implantation station. The beam is formed and shaped by apparatus located along the beam path en route to the implantation station. When operating the implanter, the interior region must be evacuated to reduce the probability of ions being deflected from the predetermined beam path as a result of collisions with air molecules.
During ion implantation a surface is uniformly irradiated by a beam of ions or molecules, of a specific species and prescribed energy. The size of the wafer or substrate (e.g. 8 inches or greater) is typically much larger than the cross-section of the irradiating beam which deposits on the wafer as a spot or xe2x80x9cribbonxe2x80x9d of about 1 inch. Commonly, in high current machines, the required uniform irradiance is achieved by moving the wafer through the beam.
Operation of an ion implanter results in the production of certain contaminant materials. These contaminant materials adhere to surfaces of the implanter beam forming and shaping structure adjacent the ion beam path and also on the surface of the wafer support facing the ion beam. Contaminant materials also include undesirable species of ions generated in the ion source, that is, ions having the either the wrong atomic mass or undesired ions of the same atomic mass.
In a conventional ion implanter 10 such as that shown in FIG. 1, an ion beam 12 is emitted from an ion source 14 and passed through a pre-analyzing magnet 16 to remove undesired types of ions. Ions having identical energies but different masses experience a different magnetic force as they pass through the magnetic field due to their differing masses thereby altering their pathways. As a result, only those desired ions of a particular atomic mass unit (AMU) are allowed to pass through a prepositioned orifice in the pre-analyzing magnet.
After passing through the pre-analyzing magnet the ion beam is accelerated to a desired energy by an accelerator 18. Negative ions are changed into positive ions by a charge exchange process involving collisions with a chemically inert gas such as argon. The positive ions then pass through a post-analyzing magnet (not shown) and a pair of vertical and horizontal scanners 20, 22 finally reach a wafer 24 where they impact the wafer 24 and are implanted.
Ion implantation has the ability to precisely control the number of implanted dopant atoms into substrates to within 3%. For dopant control in the 1014-1018 atoms/cm3 range, ion implantation is superior to chemical diffusion techniques. Heavy doping with an ion implanter, for example, can be used to alter the etch characteristics of materials for patterning. The implantation may be performed through materials that may already be in place while other materials may be used as masks to create specific doping profiles. Furthermore, more than one type of dopant may be implanted at the same time and at the same position on the wafer. Other advantages include the fact that ion implantation may be performed at low temperature which does not harm photoresist and in high vacuum which provides a clean environment.
In the convention ion implanter 10 of FIG. 1, the wafer to be implanted is clamped onto a wafer platform during the implantation process. For instance, such a wafer platform 32 is shown in FIG. 2 in a sample holding device 30 complete with a rotation mechanism 34. A perspective view of the wafer platform 32 in a horizontal position is shown in FIG. 3. After a wafer is mounted to the wafer platform 32, either by mechanical clamping or by electrostatic chucking, the wafer platform is turned by the rotational mechanism 34 into a vertical position such that ions from the ion beam emitter which are emitted in a horizontal direction may be bombarded on the surface of the wafer. The orientation of the wafer platform in the vertical direction is therefore very important for achieving a highly accurate implantation process. The criticality of the wafer platform orientation, i.e., the maintaining of a zero-angle position, becomes more important in the next generation wafer processes, such as for the 0.13 xcexcm process. It has been found that when the tilt angle of the wafer platform 32 deviates about 2xc2x0 from a zero-angle position during an ion implantation process, a variation as large as 20-30 mV in the threshold voltage value for the device produced may result. It is therefore an important process for calibrating the tilt angle of a wafer platform in order to avoid reliability issues.
It is therefore an object of the present invention to provide an apparatus for detecting the tilt angle of a wafer platform in a process machine that does not have the drawbacks or shortcomings of the conventional calibration apparatus.
It is another object of the present invention to provide an apparatus for calibrating the zero-angle position of a wafer platform in an ion implantation machine.
It is a further object of the present invention to provide an apparatus for calibrating the zero-angle position of a wafer platform in a medium current ion implanter that can be used out with high repeatability.
It is another further object of the present invention to provide an apparatus for detecting the tilt angle of a wafer platform in a process machine by utilizing laser optics, including a laser emitter and a laser receiver mounted in the machine.
It is still another object of the present invention to provide an apparatus for detecting the tilt angle of a wafer platform in a process machine by measuring the reflectance angle of a laser beam reflecting off a wafer surface that is sensitive to a change in the tilt angle.
It is yet another object of the present invention to provide a method for calibrating the zero-angle position of a wafer platform in a medium current ion implanter.
In accordance with the present invention, an apparatus and a method for detecting the tilt angle of a wafer platform are provided.
In a preferred embodiment, an apparatus for detecting the tilt angle of a wafer platform in a process machine is provided which includes a process chamber that has a cavity and a wafer platform in the cavity; a window that is substantially transparent to laser energy in a top wall of the process chamber; and a laser emitter and receiver positioned outside the process chamber juxtaposed to the window for emitting a laser beam onto a wafer positioned on the wafer platform and receiving a reflected laser beam to determined a tilt angle of the wafer platform by the intensity of the reflected laser beam.
In the apparatus for detecting the tilt angle of a wafer platform in a process machine, the laser emitter emits a continuous laser beam, or the laser emitter may be adjusted to emit a laser beam onto a portion of the wafer within 1 mm from an edge of the wafer. The process machine may be an ion implanter, or a medium current ion implanter. The wafer platform may be equipped with a mechanical clamping device, or may be equipped with an electrostatic chucking device. The window may be formed of quartz. The laser emitter may be adjusted to emit a laser beam onto a portion of the wafer that does not have active device built thereon. The process chamber may further include means for rotating the wafer platform from a horizontal loading position to a vertical test position.
The present invention is further directed to a method for detecting the tilt angle of a wafer platform in a process machine which can be carried out by the operating steps of first providing a chamber for the process machine that is equipped with a top wall, a bottom wall, and side walls connecting the top and bottom walls, and a wafer platform; then mounting a window that is substantially transparent to laser energy in the top wall; positioning a laser emitter and receiver outside the chamber juxtaposed to the window; and emitting a laser beam onto a top surface of a wafer positioned on the wafer platform and receiving a reflected laser beam by the receiver and determining an intensity of the reflected beam.
The method for detecting the tilt angle of a wafer platform in a process machine may further include the steps of providing a wafer platform that is situated in a horizontal position in the chamber, and then positioning a wafer on top of the wafer platform. The method may further include the step of emitting a continuous laser beam from the laser emitter. The method may further include the step of emitting a laser beam onto a portion of the wafer that is within 1 mm from an edge of the wafer, or the step of emitting a laser beam onto a portion of the wafer that does not have active devices built thereon. The method may further include the step of mounting a quartz window in the top wall. The method may further include the step of calculating a tilt angle from an intensity of reflected laser measured. The method may further include the step of clamping a wafer on the wafer platform by mechanical clamping means, or by electrostatic chucking means. The method may further include the steps of rotating the wafer on a wafer platform from a horizontal loading position to a vertical ion implantation position, and ion implanting a top surface of the wafer.